In the early days of computing systems, human-computer interaction (HCl) was confined to a selected group of people comprising of engineers, developers, academicians and hobbyists. With the development of personal computer in 1980s and Graphical user interfaces (GUIs), HCl has changed significantly. Traditional input devices, such as keyboard and mouse are gradually being replaced with new interaction metaphors, such as gestures, touch, and speech. Considering the new interaction paradigms, user interfaces are being re-designed to accommodate user inputs from multiple input modes. As a result, it is not uncommon to find multiple user interfaces on a computer system each corresponding to a distinct interaction modality. For instance, a speech-based input system may require a user to interact with a speech-specific interface, whereas a gesture-based interaction system may require a different mechanism. This is not a desirable situation from a user's perspective, who might want to enjoy a multimodal interaction experience in the true sense which involves using multiple input modalities at the same time in a convenient and user friendly manner.